Generally, a semiconductor device is an electronic component or structure, or a collection of such elements, which are fabricated onto a semiconductor wafer and used to carry or manipulate electrical signals. A semiconductor wafer is a thin piece of semiconductor material, such as silicon, onto which will be fabricated a large number of these electronic components. The components are interconnected to form integrated circuits. For efficiency, each wafer is typically used to form a number of dice, each of which may include millions of components. When fabrication is complete, the dice are separated and each individual die is packaged for installation into an electrical appliance such as a mobile telephone, personal computer, or MP3 player.
The fabrication of the electrical components on a wafer involves a series of process steps, most or all of which are now automated. In general, these steps include ion implantation to impart semiconductor properties to, for example, the silicon wafer, and the selective formation and removal of alternating layers of insulating and conducting material. As the individual components or structures are very small, specialized processes have been developed in order to properly form them. Although extremely small features are currently being fabricated, the popularity of compact and energy-efficient electrical appliances is compelling manufacturers to produce ever smaller and, at the same time, more capable components. New fabrication techniques are constantly needed.
A wafer surface provided for forming electronic components is sometimes referred to as a substrate. There are several types of semiconductor structures that may be formed on a semiconductor substrate. FIG. 1 is a side (elevation) view illustrating in cross section a typical semiconductor device 10. Note that as used herein, the term “device” is intended as a general term used to refer to a particular functioning component, part of a component, or collection of components. In other words, the particular device being discussed is defined by the feature or features recited, and by the context. A “semiconductor device” is one being used in a semiconductor application, and no specific properties relating to a particular component or components is implied by use of the term itself.
In the semiconductor device 10 of FIG. 1, a first transistor 11 and second transistor 16 have been formed at the surface 21 of substrate 22. In addition to a gate structure 12, first transistor 11 includes a source region 14 and a drain region 15 defining a channel 13. Likewise transistor 16 includes a gate structure 17, and a source region 18 and a drain region 20 defining a channel 19. Under certain conditions, such as the application of a charge to the respective gate structures 12 and 17, electrical current may flow through the channels 13 and 19 between the respective source and drain regions defining them. Transistors, therefore, are basically small switches that may be used for the manipulation of electrical signals. In the example of FIG. 1, transistors 12 and 16 are separated by an isolation structure 25, which has been formed in a trench 24 and is intended to prevent the operation of one of the transistors from interfering with operation of the other. This type of isolation structure is sometimes referred to as an STI (shallow trench isolation) structure. The formation of an STI may be seen more clearly in reference to FIGS. 2a through 2d. 
FIGS. 2a through 2d are a sequence of side views illustrating in cross-section the configuration of a semiconductor device 30 at various selected stages of fabrication. In FIG. 2a, it may be seen that a buffer oxide layer 34 has been formed over, and in this case directly over the substrate. The substrate may, for example, be silicon, and the oxide layer a silicon dioxide material. A photoresist layer 36 has been formed directly over the buffer oxide layer 34. Photoresist is a material that may be patterned, that is, formed into a pattern of structures and openings, because it changes composition when exposed to light. To create the pattern, the photoresist layer is selectively exposed through a mask bearing a pattern. The exposed portions either remain or are washed away (depending on the type of photoresist used) by a solvent suitable for this purpose. The remaining structures are typically used to protect the areas underneath them, allowing unprotected area to be etched themselves. When they are no longer needed, the remaining photoresist structures are removed using a solvent selected for that purpose. This process is often referred to as photolithography.
In this example, the isolation structure is formed by first forming photoresist structures 43 and 44, then etching a recess 38 into the substrate 32, and in the oxide layer 34 disposed above it. This configuration is illustrated in FIG. 2b. Once the recess 38 has been formed, an oxide material 40 is deposited, in this case filling trench 38 and covering the surrounding portions of semiconductor device 30. This configuration is shown in FIG. 2c. The portion of oxide layer 40 that is disposed within the substrate 32 portion of recess 38 is now referred to as isolation structure 39. To finish the fabrication process, the remainder of oxide layer 40 is removed, for example by a CMP (chemical metal polishing) process. The photoresist structures 43 and 44 are removed as well. The resulting configuration of semiconductor device 30, now including isolation device 39, is depicted in FIG. 2d. 
Many similar structures may be formed in this manner. For example, a structure more commonly referred to as a via is shown in FIG. 3. FIG. 3 is a side view illustrating in cross section a typical filled via structure 50. As with the recess 38 referred to above, via 50 is formed in a substrate 55 by etching a recess 54 and filling it with a fill material 53. Note that fill material 53 may be a dielectric material such as silicon dioxide or may be a low-k dielectric. It may be, and often is, a conductive material such as copper. In the example of FIG. 3, the via recess 54 is filled with a conductive fill material 53 to provide a conductor from the front side surface 51 of substrate 55 to the backside 52.
As can be seen in FIG. 3, a portion 56 of the fill material 53 in face protrudes slightly from the back side 52 so that contact may be made with a pad or other target contact area (not shown), such as one on another chip. To form this structure, the via recess 54 may have been formed, filled, and then the back side 52 etched to expose the protruding portion 56 of the fill material 53. Note that in applications like the one shown in FIG. 3, the via must often be relatively long compared to, for example, the STI 25 shown in FIG. 1, simply because it must extend from one side of the substrate to the other. The via usually cannot be as wide, either, because many such structures may be needed in a relatively limited area. The relationship if depth (length) to width is called the aspect ratio. Referring to FIG. 3, the length of the via is called out as Lv, while the width is Wv. An aspect ratio Lv/Wv of about five or greater is sometimes referred to as a high aspect ratio (A/R) via.
Unfortunately, existing methods often use materials and processes that result in unsatisfactory results. If the vias, and especially those with a high aspect ratio, cannot be properly created and filled, then their size may need to be increased to ensure proper fabrication. As mentioned above, however, what are currently in demand are small devices with a higher via density so that more components may be included in a semiconductor chip.
Needed, therefore, is a way to fabricate semiconductor devices that include vias, and especially those with high aspect ratios, which produces more reliable devices and enables higher via density while not adding inordinately to the cost of fabrication or necessitating undue and difficult process modification. The present invention provides just such as solution.